The present invention relates to an LPG (Liquefied Petroleum Gas) fuel system for a vehicle. More particularly, the invention relates to a method and a system for determining the fuel pressure and the fuel temperature inside the fuel tank of the LPG fuel system, and for estimating the LPG fuel composition based on the determined fuel pressure and fuel temperature.
Generally, an LPI (LPG injection) system used for supplying fuel to a combustion chamber includes a fuel pump that is disposed in a fuel tank that stores LPG fuel. The LPG fuel is pressurized by the fuel pump so that it is in a liquid state in the fuel supply line. The pressurized liquid fuel is injected by an injector into the combustion chamber.
Because it is more difficult to meter the LPG fuel into injectors when the LPG fuel is in a gaseous state, it is desirable to maintain the LPG fuel in the liquid state. And fuel injection control in the LPG vehicle is generally performed based on the assumption that the LPG fuel is in a liquid state.
When liquid LPG fuel vaporizes, the volume of the fuel increases by about 250 times. Therefore, if the liquid LPG fuel vaporizes during fuel injection, the amount of fuel injected into a combustion chamber is substantially decreased. To deliver the LPG fuel in the liquid state, the LPG fuel stored in the fuel tank is pressurized by the fuel pump and delivered to the injector.
In a conventional LPG vehicle, without the fuel pump, where the gaseous LPG fuel is delivered to the engine by the fuel vapor pressure itself, the lower the temperature of the LPG fuel, the less the amount of LPG fuel supplied to the combustion chamber. This causes startability and acceleration problems.
LPG is a petroleum-derived colorless gas, typically comprised primarily of either propane, a butane, or a combination of the two. LPG has been the most widely used alternative motor fuel to gasoline and diesel thus far. The butane/propane ratio of the LPG fuel is varied according to environmental situations, such as temperature. For example, it is preferable that the fuel contains 100% butane during hot summer, when the LPG fuel may be more easily evaporated because butane has a higher vaporization point. On the other hand, the butane/propane ratio of the LPG fuel is preferably 70:30 during cold winter. That is, it is preferable that the ratio of the propane is increased with a temperature decrease.
In an LPI engine that uses only LPG fuel, the amount of fuel injected and the ignition timing are determined on the basis of the liquid state of the LPG fuel when the LPG fuel tank is filled. Therefore, when liquid LPG fuel and gaseous LPG fuel coexist in the same tank, it is impossible to optimally control the fuel injection. Thus, it is desirable to determine whether the LPG fuel is in a liquid state. To determine whether the LPG fuel is in a liquid state, the LPG fuel composition must be known, that is, the butane/propane ratio of the LPG fuel must be determined.
The butane/propane ratio of the LPG fuel can be determined based on the temperature and pressure of the LPG fuel inside the fuel tank and saturation vapor pressure data. These are easily acquired by utilizing sensors detecting the temperature and the pressure of the fuel inside the fuel tank. But it is difficult to install sensors for detecting the temperature and the pressure of the LPG fuel inside the fuel tank, and furthermore, such sensors increase the cost of the fuel system.
In a preferred embodiment of the present invention, the LPG (Liquefied Petroleum Gas) fuel composition estimation method for an LPI (LPG Injection) system comprises: detecting a fuel pressure inside a fuel supply line, a fuel temperature inside the fuel supply line, a coolant temperature, and an intake air temperature at an ignition-on state; determining whether differences between the detected temperatures fall within predetermined conditions; detecting the fuel temperature and the fuel pressure inside the fuel supply line at intervals; determining a fuel temperature and a fuel pressure inside a fuel tank based on the detected fuel temperature and fuel pressure inside the fuel supply line, if it is determined that the predetermined conditions exist; and determining an LPG fuel composition from saturation vapor pressure data based on the determined fuel temperature and fuel pressure inside the fuel tank.
Preferably, the predetermined conditions comprise: a temperature difference between the fuel temperature inside the fuel supply line and the coolant temperature at the ignition-on state, a temperature difference between the fuel temperature inside the fuel supply line and the intake air temperature at the ignition-on state, and a temperature difference between the coolant temperature and the intake air temperature are less than predetermined values.
It is also preferable that the determining a fuel temperature and a fuel pressure inside a fuel tank comprises: calculating an average fuel pressure of a predetermined number of fuel pressures, detected at predetermined intervals in the fuel supply line; determining whether predetermined conditions for an estimation of a fuel pressure and a fuel temperature inside the fuel tank exist; and setting the fuel temperature inside the fuel supply line as a fuel temperature inside the fuel tank, and setting a pressure difference between the average fuel pressure and a pressure regulator control pressure as the fuel pressure inside the fuel tank, if it is determined that the predetermined conditions for an estimation exist.
It is preferable that if a difference between a current fuel temperature inside the fuel supply line and the fuel temperature inside the fuel supply line at the ignition-on state is not less than a predetermined value, it is determined that the predetermined conditions for an estimation of a fuel pressure and a fuel temperature inside the fuel tank do not exist.
It is further preferable that if a difference between a current fuel temperature inside the fuel supply line and the fuel temperature inside the fuel supply line at the ignition-on state is not less than a predetermined value, a fuel pump speed is not lower than a predetermined speed, or a difference between the calculated average pressure and a calculated average pressure of a previous routine is not less than a predetermined value, it is determined that the predetermined estimation conditions do not exist.
In another preferred embodiment of the present invention, the LPG (Liquefied Petroleum Gas) composition estimation system for an LPI system comprises: a fuel tank, a fuel pump, an injector, a pressure regulator, a detecting unit, and a control unit. In this embodiment, the fuel tank stores LPG fuel. The fuel pump is disposed inside the fuel tank. The fuel pump pressurizes the LPG fuel. The injector is connected to the fuel pump via a fuel supply line. The pressure regulator is disposed in a fuel return line connecting the injector and the fuel tank. The detecting unit detects one or more engine operating parameters and generates corresponding signals. And the control unit receives the signals of the detecting unit and estimates an LPG fuel composition. The control unit being programmed to execute a method comprising: determining whether differences between the detected temperatures fall within predetermined conditions; detecting the fuel temperature and the fuel pressure inside the fuel supply line at intervals; determining a fuel temperature and a fuel pressure inside a fuel tank based on the detected fuel temperature and fuel pressure inside the fuel supply line, if it is determined that the predetermined conditions exist; and determining an LPG fuel composition from saturation vapor pressure data based on the determined fuel temperature and fuel pressure inside the fuel tank.
In a further preferred embodiment of a detecting unit, an ignition detector detects whether a current state is an ignition-on state, an engine-operation state, or an engine-off state. A fuel pressure detector detects a fuel pressure inside a fuel supply line. A fuel temperature detector detects a fuel temperature inside the fuel supply line. A coolant temperature detector detects a temperature of a coolant. An intake air temperature detector detects a temperature of intake air. And a fuel pump speed detector detects a speed of the fuel pump.
An additional preferred embodiment of the method of the present invention includes detecting an LPG temperature and determining whether the LPG temperature has stabilized, detecting an LPG pressure and determining whether an LPG pressure has stabilized, determining an LPG fuel tank temperature based on the detected LPG temperature and determining an LPG fuel tank pressure based on the detected LPG pressure, and estimating and LPG fuel composition based on the LPG fuel tank temperature and the LPG fuel tank pressure.